1. Field of the Invention
The present invention relates to an image processing apparatus that processes digital image data, and more particularly to an image processing apparatus that embeds additional information in the digital image data in superimposed form.
2. Description of the Prior Art
Techniques are available for embedding additional information in digital image data in superimposed form. Recently, there is an active movement to use the additional data embedding techniques for copyright protection or illegal copy prevention of digital publications such as still image data. Also, there is a movement to use the additional data embedding techniques for the purpose of integrating specific electronic image data with other digital image data related to it, or secret communications.
When the additional data embedding techniques are used for the above purposes, additional data such as a copyright ID, user ID, or any identification data is embedded in image data for distribution so that they are visually inconspicuous. There are two known examples of such additional data embedding techniques. One is the technique (disclosed by Published Japanese Translation of PCT International Publication for Patent Application No. Hei 9-509795) for embedding additional data by superimposing noise signals representing additional data in image data. The other is the technique (Japanese Published Unexamined Patent Application No. Hei 10-51650) for embedding additional data by subjecting image data to Fourier transform and manipulating transform coefficients in a concentric region indicative of identical frequencies on a frequency space.
Since these additional data embedding techniques are primarily used for the purpose of preventing electronic illegal copy and illegal use of electronic image data, the additional data is embedded in the image data so that an image based on the electronic image data is displayed on a display apparatus with no reduction in the image quality. Hence, once the electronic image data has been printed by a printer, it is difficult to read the additional data embedded in the image data from the image formed on paper after printing.
Therefore, even if additional data is embedded in electronic image data, since the additional data cannot be recognized from the printed-out image, the printed-out image may be copied and distributed, with the result that the copyright of the electronic Image data is infringed. To read the additional data from the printed-out image, it is conceivable to embed the additional data in the electronic image data to be distributed, with a higher embedding intensity. However, this approach poses the problem of heavily reducing the image quality of the electronic image data displayed on a display apparatus.
On the other hand, as a conventional technique for embedding additional data in a printed-out image, there is a technique for embedding additional data in a printed image by superimposing patterns having positive and negative amplitudes on the image in yellow ink (Japanese Published Unexamined Patent Application No. Hei 6-113107). However, this technique has the problem that, since no pattern is embedded if all bits have a value of “0” as additional data to be embedded, it is difficult to judge whether additional data is not embedded or additional data with all bits having a value of “0” is embedded.
As another conventional technique for embedding additional data in a printed image, there is a technique for embedding additional data by code patterns with two types of minute slant patterns associated with additional data bits of “0” and “1” so that the code patterns are detected from an image by a scanner (Japanese Publish Unexamined Patent Application No. Hei 4-233677). However, this technique has the problem that, since the code patterns are embedded in black in a white region of the image, the embedded additional data is visible.
Although it is conceivable to embed additional data in a predetermined color component of a color image, it becomes difficult to detect an embedded pattern, depending on a local property (density) of the image. Accordingly, if pattern intensity is increased to enable detection in high-density portions of the image, there might arise the problem that the embedded pattern becomes conspicuous in low-density portions.